The structure of Zr-2.3%Nb and Zr-5.5%Nb alloy martensites on tempering at different temperatures in the range of 350 to 600°C was studied by optical and transmission electron microscopy. The equilibrium β-niobium phase (β2) was found to be the precipitating phase on tempering the Zr-2.3%Nb martensites at temperatures up to 500°C and the Zr-5.5% Nb martensites at temperatures upto 450°C. Precipitation of the metastable β1 phase of the monotectoid composition (Zr20%Nb) was observed to occur on tempering the Zr-2.3%Nb alloy at 550 and 600°C and the Zr-5.5%Nb alloy at 500 and 550°C. On tempering the latter alloy at 600°C, the martensite was found to revert back to the supersaturated β phase, which subsequently decomposed into a mixture of the α and the β1 phases. These observations have been explained on the basis of hypothetical free energy versus composition diagrams. The_orientation relation of the β1 precipitates with respect to the α phase was found to be as follows: (000l)β ?{011}β1; (1120)β ?<111>β1. It was also seen that a β1 precipitate forming at a twin boundary maintains equivalent orientation relations with the two adjacent twin related portions.
This paper was originally published in Journal of Nuclear Materials 62 (1976) Pages 229 - 239
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